1. Field of the Invention
The present invention is directed to communication systems and networks and is particularly directed to such systems and networks which use multi-carrier protocols such as orthogonal frequency division multiplexing and discrete multi-tone protocols, and to techniques for communicating thereover.
2. Background of the Related Art
Orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT) are two closely related formats which have become popular as communication protocols. Systems of this type take a relatively wide bandwidth communication channel and break it into many smaller frequency sub-channels. The narrower sub-channels are then used simultaneously to transmit data at a high rate. These techniques have advantages when the communication channel has multi-path or narrow band interference.
The following discussion of the prior art and the invention will address OFDM systems; however, it will be understood that the invention is equally applicable to DMT systems (as well as other types of communication systems) with only minor modifications that will be readily apparent to those skilled in the art.
A functional block diagram of a typical OFDM transmitter is shown in FIG. 1. Here, an incoming stream 10 of N symbols d0, d1 . . . dNxe2x88x921 is mapped by a serial-to-parallel converter 20 over N parallel lines 30, each line corresponding to a particular subcarrier within the overall OFDM channel. An Inverse Fast Fourier Transform circuit 40 accepts these as frequency domain components and generates a set 50 of time domain subcarriers corresponding thereto. These are converted by a parallel-to-serial converter 60. Due to the characteristics of the inverse Fourier transform, although the frequency spectra of the subcarrier channels overlap, each subcarrier is orthogonal to the others. Thus, the frequency at which each subcarrier in the received signal is evaluated is one at which the contribution from all other signals is zero.
A functional block diagram of the corresponding OFDM receiver is shown in FIG. 2. Here, an OFDM signal is received and converted into multiple time domain signals 210 by a serial-to-parallel converter 220. These signals are processed by a Fast Fourier Transform processor 230 before being multiplexed by parallel-to-serial converter 240 to recover the original data stream 250.
Systems such as OFDM and DMT systems either do not share the main channel with other users at all (e.g., when they are implemented using a telephone modem), or share the channel in time (e.g., when implemented in TDMA and CSMA schemes); thus, their flexibility and ease of use is limited. Sharing the channel in time (i.e., allowing only one user to transmit at a time) has two serious disadvantages. First, to maintain high throughput, all nodes sharing the channel must operate at a high data rate, and therefore be equally complex; thus, no less-complicated processing circuitry which might otherwise be used with low data rate channels can be employed. Second, a user who actually desires a low data rate must send data as very short high speed bursts over the network. In order to overcome propagation loss in the path, such a node must transmit at a high peak power because the transmit power is proportional to the peak data rate. Again, economies inherent in the low data rate processing cannot be exploited.
As a practical example, the IEEE 802.11a communication standard specifies transmission with 52 sub-channel frequencies. This requires substantial signal processing; a high transmit power while active to achieve significant range; a large peak-to-average ratio while actively transmitting; high resolution ADCs and DACs; and very linear transmit and receive chains. While such complicated hardware allows transmission up to 50 Mb/s, this level of performance is overkill for something like a cordless phone, which only requires roughly a 32 kb/s transmission rate.
In connection with the peak-to-average ratio, note that for 52 sub-channels, while transmitting the peak-to-average ratio of the signal is 522/52=52 in power. Therefore, to avoid distortion of the signal, the power amplifier must be substantial enough to provide far more instantaneous power than is required on average. Since the peak-to-average ratio is directly proportional to the number of sub-channels, building a lower capacity unit that uses fewer carriers can substantially decrease the costs of such a device.
In an effort to solve the above shortcomings of the prior art, a system has been proposed which implements frequency communication but allows channel sharing between users in a way that would allow simple nodes such as a 32 kb/s cordless phone to transmit continuously at a low rate while other high speed nodes such as 20 Mb/s video streams communicate at a much higher data rate simultaneously. This can be an OFDM or DMT system in which the simple nodes are allowed to transmit continuously on one or just a few of the frequency sub-channels, while the other nodes avoid putting any signal into those sub-channels.
A system such as the one described above advantageously allows individual carriers to be used by different devices in a multi-carrier communication network such as an OFDM network to allow the simultaneous communication of high data rate devices which use many carriers and low data rate devices which use only one or a few carriers. Devices that use one or a few carriers can be lower in cost and power consumption than their counterparts which use many carriers due to the reduced digital complexity, reduced analog accuracy, reduced required transmit power and reduced peak-to-average power ratios of their communication sections. By allowing the low data rate and high data rate nodes to communicate at the same time, the overall throughput of the network remains high.
There are a number of constraints imposed on the physical layer of almost any communication network in which low-cost implementation and support for multimedia applications are desired. For example, it is more inconvenient, difficult, expensive and time-consuming to construct such communication systems that can simultaneously transmit and receive, particularly in wireless domains. Therefore, it is best to coordinate nodes in communication networks so that no node is required to simultaneously transmit and receive, i.e., to implement half duplex operation.
Further, because of the great attenuation, multi-path propagation and variability of certain communication media such as radio channels, signals transmitted therethrough must live with high error rates. This frequently causes data packets to contain errors. It is important for the device to somehow signal if a packet arrived with an error so that the transmitting node can resend the packet. Also, multimedia traffic requires low latency and low jitter in the arrival of packets. Interactive traffic such as telephony and video telephony are sensitive to both latency and jitter. Streaming applications such as CD-quality audio and digital video are primarily sensitive to the jitter in the time of arrival of subsequent packets of information. Guaranteeing the quality of service by insuring regular access to the medium and preventing collisions is the main goal of the protocol.
In addition to physical layer constraints common to low-cost, multimedia-capable communication systems, there are a number of constraints imposed on the physical layer of the network in order to support overlaid multi-carrier operation. For example, nodes in such overlaid multi-carrier systems preferably precompensate their transmit frequencies. This is because the close carrier spacings used in multi-carrier modulation make it difficult to insure that a given node will transmit its carriers with sufficient frequency accuracy to ensure that adjacent carriers do not bleed over into one another. If frequencies are sufficiently off, two nodes may actually end up transmitting their signals at the same frequency. Ensuring sufficient accuracy by using extremely accurate frequency references is in most cases prohibitively expensive. Instead, the nodes should preferably compensate for the inaccuracy of their transmitting frequency before transmitting.
Systems have been proposed which do this by, e.g., locking the frequency used by each node to a highly accurate external reference such as a Global Positioning System (GPS) satellite; locking the frequency used by each node to the transmit frequency of the base station; or adjusting the frequency used by each node according to closed-loop feedback signals sent by the base station.
Another physical layer constraint in overlaid multi-carrier systems is that nodes should adjust their transmissions in time so that when the signals from different nodes arrive at a receiver, the symbol transitions in the signal streams are aligned in time within a small fraction of the total symbol time. This is once again due to the close orthogonal spacing of the carriers in OFDM and DMT systems. The signals are not orthogonal during the portion of the symbols in which the symbols are making transitions. Therefore, separating the signals from different nodes on different carriers becomes difficult if some of the signals are making symbol transitions while others are stationary. Systems have been proposed which address this constraint by, e.g., adjusting the transmission of packets at the nodes according to a highly accurate external time reference such as the GPS satellite mentioned above; adjusting the transmission of packets at the nodes according to closed-loop feedback signals sent by the base station; or simply relying on the nodes"" close proximity or nearly equal distance to the base station to ensure there is not a significant amount of delay in their transmitted signals.
Further, overlaid multi-carrier OFDM and DMT systems theoretically use perfectly orthogonal carriers; however, in practice some crosstalk between carriers is present. This may be due to distortion in the analog domain, limited resolution in the digital domain, or frequency inaccuracies. Because the carriers will not be perfectly orthogonal, it is important that carriers from different nodes arrive at the receiver with similar power levels. Too great a power difference, coupled with imperfect orthogonality, will make it impossible to separate the signals. A system has been proposed which addresses this constraint by implementing a closed-loop power control scheme in which the strength of each signal is adjusted at the node according to feedback signals sent to it by the base station, or by implementing an open-loop power control scheme in which the strength of each signal is adjusted at the node according to the power level of the base station signal it receives.
The above constraints on overlaid multi-carrier systems favor a communication system where in one period of time multiple nodes transmit to a base station, and during another period of time the base station transits to the multiple nodes. It is difficult to meet time alignment and power control at all nodes simultaneously if multiple peer-to-peer style communications are allowed at once.
Existing communication protocols do not provide for the use of carriers within a multi-carrier communication system such as an OFDM or DMT system. In addition, existing communication protocols do not respect the constraints listed above. While all of these constraints are not hard and fast, protocols that do not respect these constraints result in communication systems that are more expensive, less efficient, less robust or give poorer quality service.
In view of the above problems of the prior art, an object of the present invention is to provide an overlaid, multi-carrier communication system which implements effective self-adjustment of its operating frequencies.
It is another object of the present invention to provide an overlaid, multi-carrier communication system which implements effective self-control of its communication timings.
It is a further object of the present invention to provide an overlaid, multi-carrier communication system which implements effective power control of individual carrier transmit powers.
It is still another object of the present invention to provide an overlaid, multi-carrier communication system in which no node is required to simultaneously transmit and receive.
It is an even further object of the present invention to provide an overlaid, multi-carrier communication system which provides an effective acknowledgement and retransmission mechanism for communicated packets.
It is yet another object of the present invention to provide an overlaid, multi-carrier communication system which exhibits low latency and jitter in the arrival of packets.
The above objets are achieved according to one aspect of the present invention by providing an overlaid multi-carrier communication system which is able to assign carriers and time slots to meet the communication requirements of each node with few or no collisions. Simple low data rate nodes are allowed to use a small number of sub-carriers while more complicated nodes use the remainder, and preferably functionality is provided to ensure that adjacent sub-channels are reliably spaced apart in frequency so that they do not bleed over into one another; to ensure that signals from all nodes arrive at the base station with well-aligned symbol transitions; and to ensure that signals from the various nodes arrive at the base station with similar power levels.